what is an "illuminati"

Tweet

hello everyone~ this word is new to me~ i just want to ask if what it totally means? i do not know if it is bad or good, i need personal reviews today, i just do not understand the word itself, its meaning, its approach to us or something. one of my classmate just shouted about that word. can my fellow mylotters enlighten me to what it really means? thank you everyone and have a good day (^^,)